Today mobile broadband services over cellular systems are becoming more and more common. One underlying reason is the introduction of High Speed Packet Access (HSPA) radio bearers in live networks. There is also an increased interest in IP Multimedia Subsystem (IMS) based services, such as Voice over IP (VoIP), and Push to talk over Cellular (PoC).
HSPA introduces the possibility of downloading and uploading data with a speed of several Mbits/s, but there is also standardization work ongoing in 3GPP to boost VoIP capacity.
In High Speed Downlink Packet Access (HSDPA) a shared channel is employed. The use of a shared channel results in that several channelization codes are shared between users on a per 2 ms TTI basis for transmission.
In HSDPA the basic shared channel structure includes a number of codes, for example 8, which are available for High Speed-Downlink Shared Channel (HS-DSCH) transmission every 2 ms TTI. A user can use all codes, such as all 8 if 8 codes are available, or the codes can be divided between users during the 2 ms Transmission Time Interval (TTI). Dividing codes between different users is usually referred to as code multiplexing, i.e. the users are multiplexed on the same TTI by being assigned different codes. The UMTS Terrestrial Radio Access Network (UTRAN) assigns a number of High Speed Shared Control Channels (HS-SCCHs) to match the number of code multiplexed users.
Although a mobile station is only required to be able to listen to a maximum of 4 HSSCCHs more than HS-SCCHs can be deployed per cell. The HS-SCCH carries information about which channelization codes that a mobile station is to decode, the User Equipment (UE) identity, which identifies the receiver of the information, Hybrid Automatic Repeat Request (HARQ) parameters and Transport Format and Resource Combination (TFRC) i.e. modulation scheme, channelization code set and transport block size, etc.
Studies show that in order to allocate many VoIP calls in a single cell more than 4 HS-SCCHs must be used. By allocating more than 4 HS-SCCHs per cell and spread out the UEs on these in a smart way higher code multiplexing than 4 can be used. However, as the VoIP service is comparably low rate service the HS-SCCH overhead may be high. Therefore a study item in 3GPP called continuous packet connectivity (CPC) investigated several schemes to improve services like VoIP, with focus on reducing overhead, see [3GPP TR25.903].
One improvement was the so called HS-SCCH less operation mode. Basically, in HS-SCCH less mode, the HS-SCCH overhead can be reduced by simply being removed or introducing discontinuous transmission. The disadvantage is that only a few transport format combinations (TFC). i.e. packet sizes, can be used which are semi-static and configurable per UE. The UE makes a so called blind decoding assuming the configured packet sizes. In the currently proposed standard, two (2) different Transport Block (TB) sizes are allowed in HS-SCCH less operation, and four (4) different TB sizes are allowed in Reduced Complexity HS-SCCH-less operation. If other TFCs or TB sizes are needed than these two or four, then the normal High Speed Dedicated Physical Control Channel HS-DPCCH must be used. For VoIP this is not a critical limitation.
In normal operation mode, the network receives feedback information such as channel quality indications and ACK/NACKs, on the uplink channel HS-DPCCH. The network utilizes that information in the downlink scheduling decision and in the HARQ process. The downlink HS-SCCH indicates to the user which HS-DPSCH it shall decode, the HARQ process number and a CRC. Both parts employ a terminal specific masking, which is used by the mobile station/terminal to determine that the data is actually intended for it. Finally the mobile station/terminal despreads the data sent on the HS-PDSCHs.
If HS-SCCH less operation is used, each VoIP user in HS-SCCH less operation is assigned a specific code, or HS-PDSCH channel, at the start of the session. Hence, the code assignment is highly important, and care should be taken to avoid code blocking as the mobile stations in HS-SCCH less mode only try blind decoding on the given pre-defined HS-PDSCH code, i.e. channelization code. It is of course possible to change this code, but not instantaneously since it requires an RRC message, see 3GPP TS 25.308.
Code blocking is when the scheduler assigns two or more users to transmit in the same TTI, but since they use the same HS-PDSCH code this is not possible. In normal operation the code assignment is done on the fly by using the HS-SCCH to point out the HS-PDSCH code.
Today there exist no solution to handle this situation. One solution could be to assign x out of C possible HS-PDSCH codes, for all VoIP users, and all other codes (C-x) for other (non CPC) services, such as web browsing, etc. It should be noted that the network cannot transmit during the same TTI to users in HS-SCCH less operation which are assigned the same code.
However, when the number of VoIP users increases in the cell such a solution is less efficient. When the number of VoIP users increases and dominates the traffic in cell, the probability to simultaneously transmit to many VoIP users increases. And since the code limit is fixed, there is always a chance that the need for codes exceeds that code limit. Without HS-SCCH less operation and CPC. 4 users per TTI was typically the code limit. This since the standard stipulated that each UE only listens up to 4 groups of codes (HS-SCCH channels). Furthermore, it is not obvious how to spread the users among the codes available.
Hence, there exist a need for a method and a system that is able to handle many VoIP calls in a single cell.